An adaptive controller for self-regulating, delayed processes is described in German Patent No. OS 39 29 615. This adaptive controller has the ability to automatically adjust its parameters to the characteristics of a controlled system which vary over time. To do this, the process response to an abrupt variation in the setpoint is recorded, and an iterative method is used to find a PTn model which can emulate this step response as accurately as possible. Optimum parameters are those of the PTn model in which the error between the step responses of the PTn model and those of the real process, calculated by the method of least error squares, is minimal. Upon completion of process identification, a controller is designed according to the optimum absolute value using the identified process model. Single-loop control loops with PID controllers are a possible controller type. The processes to be controlled are self-regulating, which means that they represent a controlled system in which the step response enters a new steady state.
A disadvantage of this conventional method lies in the fact that, when turning on the conventional adaptive controller, it is often necessary to perform a time-consuming step change test before the controller can be optimized. This test is not completed until the controlled variable once again approaches the new steady-state limit after the setpoint step change. In doing this, the steady-state criterion must be carefully selected, since the identification process will take up too much time if the criterion is too narrowly defined, while too broad a criterion will increase the risk of incorrect identification. In addition, the control quality that can be achieved with a linear PID controller is in principle limited, since a fast start control response is achieved at the price of overshooting the controlled variable.
Another disadvantage of the conventional identification method lies in the creeping time constants, i.e. poles which are very close to the origin of the Laplace plane and can cause the process to behave very much like an integrator, so that a steady state cannot be achieved for a very long time following a step change in the manipulated variable. Consequently, the conventional identification method based on a complete step response is very time-consuming in this type of process.